You have a cat who is curious. If I had a dollar for every time I've had to throw out a drink or had a drink knocked off a coffee table, etc,.....
The answers are 1) use a covered stainless steel or BPA free water bottle for all of your drinks. Some of them come with wide necks, some are big plastic cups with lids/straws. (2) cover your drink with a Kleenex and pray. I'm usually the lazier type but Kleenex seems to work.
If your cat is going around licking condensation off of windows, drinking the soapy water out of the bottom of the tub, seems to have endless thirst and big pees, then he needs to go to the vet. That could be kidney damage/failure and the sooner you catch it the easier it is for your cat to live a healthy life.
I have a good sized pet water fountain that would work for dogs or cats, and has 2 tiers; that seems to keep all three of my cats happier now.
Really be careful about letting them near any chocolate, caffeinated or grape juice drinks, for example. Toxic. Milk can also make your cat pretty sick.